Enhanced dehydrogenation performance of LiBH4 by confinement in porous NiMnO3 microspheres

The dehydrogenation behavior of LiBH₄ has been investigated when confined into porous NiMnO₃ microsphere via a wet chemical impregnation method. The confinement of LiBH₄ in the pores of NiMnO₃ nanoparticles leads to a significant decrease of the onset and the maximum desorption temperatures. The composites begin to release hydrogen at 150 °C and the maximum desorption temperature is 300 °C, which are much lower compared to the raw LiBH₄. Also, the hydrogen release amount is found to be increased. Moreover, the LiBH₄@NiMnO₃ composites exhibit excellent dehydrogenation kinetics, with 2.8 wt% hydrogen released in 1 h at 300 °C. X-ray diffraction and Fourier transform infrared spectroscopy are used to deduce the desorption mechanism of NiMnO₃.     

A synergistic effect between nanoconfinement of carbon aerogels and catalysis of CoNiB nanoparticles on dehydrogenation of LiBH4

A synergistic effect of nanoconfinement and catalyzing is a new strategy to enhance the dehydrogenation properties of complex hydrides. Herein, LiBH₄ has been infiltrated into a CoNiB-loaded carbon aerogels system (donated as LiBH₄@CA@CoNiB). It is found that the desorption performances of LiBH₄ are significantly strengthened. The onset desorption temperature of LiBH₄@CA@CoNiB is decreased to 192 °C, and majority of the liberation occurs at about 320 °C, much lower than that of pure LiBH₄. Also, about 15.9 wt% H₂ could be released below 600 °C. Furthermore, LiBH₄ doped with CA@CoNiB exhibits an excellent desorption kinetics, with a capacity of 9.33 wt% H₂ released in 30 min at 350 °C, while only 2.13 wt% H₂ is gained for bulk LiBH₄. In addition, the apparent activation energy (Ea) is reduced sharply from 59.00 kJ/mol (pure LiBH₄) to 46.39 kJ/mol.     

Recent progress in magnesium hydride modified through catalysis and nanoconfinement

Hydrogen is an ideal energy carrier because of its high chemical energy, environmental friendliness and renewability. In order to realize the safe, efficient and compact hydrogen storage, various solid-state hydrogen storage materials based on the physisorption or chemisorption of hydrogen have been developed over the past decades. Among them, magnesium hydride, MgH₂, is identified as one of the most promising candidates due to its high hydrogen storage density, low cost and abundance of Mg element. However, the sluggish kinetics and high thermodynamic stability of MgH₂ result in its high operation temperature and low hydrogen sorption rate, impeding its practical application. In this article, the recent progress in catalysis and nanoconfinement effects on the hydrogen storage properties of MgH₂ is comprehensively reviewed. In particular, the synergetic roles of catalysis and nanoconfinement in MgH₂ are highlighted. Furthermore, the future challenges and prospects of emerging research for MgH₂ are discussed. It is suggested that the nonmetal-doped porous carbon materials could be a class of ideal additives to enhance the hydrogen storage properties of MgH₂ by the synergetic effects of catalysis and nanoconfinement.     

Nanoconfinement effects on hydrogen storage properties of MgH2 and LiBH4

To find a solution to efficiently exploit renewable energy sources is a key step to achieve complete independence from fossil fuel energy sources. Hydrogen is considered by many as a suitable energy vector for efficiently exploiting intermittent and unevenly distributed renewable energy sources. However, although the production of hydrogen from renewable energy sources is technically feasible, the storage of large quantities of hydrogen is challenging. Comparing to conventional compressed and cryogenic hydrogen storage, the solid-state storage of hydrogen shows many advantages in terms of safety and volumetric energy density. Among the materials available to store hydrogen, metal hydrides and complex metal hydrides have been extensively investigated due to their appealing hydrogen storage properties. Among several potentials candidates, magnesium hydride (MgH₂) and lithium borohydride (LiBH₄) have been widely recognized as promising solid-state hydrogen storage materials. However, before considering these hydrides ready for real-scale applications, the issue of their high thermodynamic stability and of their poor hydrogenation/dehydrogenation kinetics must be solved. An approach to modify the hydrogen storage properties of these hydrides is nanoconfinement. This review summarizes and discusses recent findings on the use of porous scaffolds as nanostructured tools for improving the thermodynamics and kinetics of MgH₂ and LiBH₄.     

Ultra-fast dehydrogenation behavior at low temperature of LiAlH4 modified by fluorographite

LiAlH₄ modified by different weight ratios of fluorographite (FGi) can be synthesized through mechanical ball-milling and their dehydrogenation behaviors were investigated. LiAlH₄ particles distributed on the FGi surface with greatly decreased sizes are achieved, comparing with ball-milled pristine LiAlH₄. Greatly reduced dehydrogenation temperatures are discovered in LiAlH₄-FGi composites. Among these composites, LiAlH₄-40FGi composite exhibits an ultra-fast hydrogen release at very low temperature as 61.2 °C, and 5.7 wt% hydrogen is liberated in seconds. Besides, the released hydrogen is of high purity according to MS test. Furthermore, XRD analysis on the dehydrogenated products proves that FGi changes the dehydrogenation reaction pathway of LiAlH₄, through which the dehydrogenation reaction enthalpy change is remarkably reduced, leading to greatly improved hydrogen desorption properties. Such investigations have discovered the potential of solid-state way of producing hydrogen under ambient temperatures.     

Hydrogen storage properties of activated carbon confined LiBH4 doped with CeF3 as catalyst

CeF₃ as a catalyst is first added to activated carbon (AC) by ball milling under low rotation speed. Then the treated AC was used as the scaffold to confine LiBH₄ by melt infiltration process. The combined effects of confinement and CeF₃ doping on the hydrogen storage properties of LiBH₄ are studied. The experimental results show that LiBH₄ and CeF₃ are well dispersed in the AC scaffold and occupy up to 90% of the pores of AC. Compared with pristine LiBH₄, the onset dehydrogenation temperature for LiBH₄-AC and LiBH₄-AC-CeF₃ decreases by 150 and 190 °C, respectively. And the corresponding dehydrogenation capacity increases from 8.2 wt% to 13.1 wt% for LiBH₄-AC and 12.8 wt% for LiBH₄-AC-CeF₃, respectively. The maximum dehydrogenation speed of LiBH₄-AC and LiBH₄-AC-CeF₃ is 80 and 288 times higher than that of pristine LiBH₄ at 350 °C. And LiBH₄-AC andLiBH₄-AC-CeF₃ show good reversible hydrogen storage properties. On the during 4th dehydrogenation cycle, the hydrogen release capacity of LiBH₄-AC and LiBH₄-AC-5 wt% CeF₃ reaches 8.1 and 9.3 wt%, respectively.     

The destabilization of LiBH4 through the addition of Bi2Se3 nanosheets

Efficient storage of hydrogen is a key issue to establish hydrogen infrastructure. In the efforts of searching suitable hydrogen storage alloys, several systems have been explored so far. All of them suffers from some drawbacks such as low gravimetric capacity, high stability, slow sorption kinetics, etc. Lithium borohydride (LiBH₄) is one of the leading contender among the hydrogen storage materials owing to its high hydrogen content of 18.5 wt%. However, its high stability needs a high operating temperature (>450 °C) for the decomposition. Recently, a thermochemical reaction between Bi₂X₃ and LiBH₄ was observed at 120 °C while performing experiments on the anode properties of Bi₂X₃ (X = S, Se, & Te) for Li-ion batteries. This indicated the possibility of destabilization of LiBH₄ and its low-temperature decomposition. This work presents the effect of Bi₂Se₃ addition to the decomposition properties of LiBH₄ using XRD and XPS techniques. The first step decomposition was observed to be initiated at around 180 °C, which is much lower than 450 °C for the pristine LiBH₄. A further reduction in the onset temperature is observed when the bulk Bi₂Se₃ is replaced by the nanosheets of this material. The mechanism of this destabilization is reported herein.     

Improved reversible dehydrogenation properties of MgH2 by the synergetic effects of graphene oxide-based porous carbon and TiCl3

In this study, we used a combination of graphene oxide-based porous carbon (GC) and titanium chloride (TiCl₃) to improve the reversible dehydrogenation properties of magnesium hydride (MgH₂). Examining the effects of GC and TiCl₃ on the hydrogen storage properties of MgH₂, the study found GC was a useful additive as confinement medium for promoting the reversible dehydrogenation of MgH₂. And TiCl₃ was an efficient catalytic dopant. A series of controlled experiments were carried out to optimize the sample preparation method and the addition amount of GC and TiCl₃. In comparison with the neat MgH₂ system, the MgH₂/GC-TiCl₃ composite prepared under optimized conditions exhibited enhanced dehydrogenation kinetics and lower dehydrogenation temperature. A combination of phase/microstructure/chemical state analyses has been conducted to gain insight into the promoting effects of GC and TiCl₃ on the reversible dehydrogenation of MgH₂. Our study found that GC was a useful scaffold material for tailoring the nanophase structure of MgH₂. And TiCl₃ played an efficient catalytic effect. Therefore, the remarkably improved dehydrogenation properties of MgH₂ should be attributed to the synergetic effects of nanoconfinement and catalysis.     

Enhanced dehydrogenation performance of ammonia borane con-catalyzed by novel TiO2(B) nanoparticles and bio-derived carbon with well-organized pores

A novel TiO₂(B) confined in porous bio-derived carbon has been prepared for dehydrogenated catalyzation of NH₃BH₃. The microstructural characterizations of as-prepared samples show that the nanoconfinement in well-organized micro/mesopores of carbon can avoid the aggregations of TiO₂(B) nanoparticles and NH₃BH₃. The dehydrogenation measurement demonstrates the dehydrogenated thermodynamic and dynamic properties of NH₃BH₃ could be improved under the con-catalyzation of TiO₂(B) and porous carbon. The results suggest that both TiO₂(B) and porous bio-derived C are promising catalysts. Additionally, it also provides a high-value solution for the disposal of agricultural wastes.     

Hydrogen energy,economy and storage: Review and recommendation

The hydrogen economy is a proposed system where hydrogen is produced and used extensively as the primary energy carrier. Successful development of hydrogen economy means innumerable advantages for the environment, energy security, economy, and final users. One major key to wholly develop hydrogen economy is safe, compact, light and cost-efficient hydrogen storage. The conventional gaseous state storage system as pressurized hydrogen gas and liquid state storage system pose safety and cost problems to onboard applications; therefore, they do not satisfy the future goals for a hydrogen economy. Fortunately, solid-state storage systems based on metal hydrides have demonstrated great potentials to store hydrogen in large quantities in a quite secure, compact, and repeatedly reversible manner and thus, becoming increasingly attractive option for hydrogen applications. However, techno-economic feasibility of hydrogen storage systems is yet to be realized as none of the current metal hydrides fulfill all the essential criteria for a practical hydrogen economy, mainly because of low hydrogen storage capacity, sluggish kinetics and unacceptable temperatures of hydrogen absorption/desorption. This article gives a brief review of hydrogen as an ideal sustainable energy carrier for the future economy, its storage as the stumbling block as well as the current position of solid-state hydrogen storage in metal hydrides and makes a recommendation based on the most promising novel discoveries made in the field in recent times which suggests a prospective breakthrough towards a hydrogen economy.     

Synergistic effect of metal components of the low-loaded Pt-Ni-Cr/C catalyst in the bicyclohexyl dehydrogenation reaction

The problem of hydrogen storage in liquid organic hydrogen carriers is not only the choice of an appropriate organic substrate, but the development of a selective and active catalyst containing as low as possible noble metals. A synergistic effect of increasing conversion and selectivity in bicyclohexyl dehydrogenation to biphenyl on trimetallic Pt-Ni-Cr/C catalysts with an extremely low Pt loading (0.1 wt %), compared with bimetallic Ni-Cr/C and Pt/Ni/C systems, due to the supporting of platinum on nickel-chromium nanoparticles was established for the first time. The TOF values (mmol (H₂)/g_Pt min) for hydrogen evolution under conditions of the reaction of bicyclohexyl dehydrogenation (320 °C, 0.1 MPa) on Pt supported onto a Ni-Cr/С composite exceed by two orders of magnitude the values found for the two-component catalysts. The maximum amount of the evolved hydrogen correlates to the selectivity of the complete dehydrogenation of bicyclohexyl into biphenyl on the Pt-Ni-Cr/C catalyst. The formation of a Ni-Cr solid substitution solution in a Ni-Cr composite deposited on a carbon carrier is shown by magnetometry, XRD, and TEM methods.     

Zn-MOF assisted dehydrogenation of ammonia borane: Enhanced kinetics and clean hydrogen generation

We report controllable and enhanced hydrogen release kinetics at reduced temperatures in ammonia borane (AB) catalyzed by Zn-MOF-74. AB is loaded into the unsaturated Zn-metal coordinated one-dimensional hexagonal open nanopores of MOF-74 (ABMOF) via solution infiltration. The ABMOF system provides clean hydrogen by suppressing the release of detrimental volatile byproducts such as ammonia, borazine and diborane. These byproducts prevent the direct use of AB as a hydrogen source for polymer electrolyte membrane fuel cell applications. The H₂ release temperature, kinetics, and byproduct generation are dependent on the amount of AB loading. We show that nanoconfinement of AB and its interaction with the active Zn-metal centers in MOF are important in promoting efficient and clean hydrogen generation.     

Compaction of LiBH4-LiAlH4 nanoconfined in activated carbon nanofibers: Dehydrogenation kinetics,reversibility, and mechanical stability during cycling

To enhance volumetric hydrogen capacity for on-board fuel cells, compaction of LiAlH₄-LiBH₄ nanoconfined in activated carbon nanofibers (ACNF) is for the first time proposed. Loose powders of milled and nanoconfined LiAlH₄-LiBH₄ samples are compacted under 976 MPa to obtain the pellet samples with thickness and diameter of ～1.20–1.30 and 8.0 mm, respectively. Dehydrogenation temperature of milled LiAlH₄-LiBH₄ increases from 415 to 434 °C due to compaction, while those of both compacted and loose powder samples of nanoconfined LiAlH₄-LiBH₄ are lower at comparable temperature of 330–335 °C. Hydrogen content liberated from milled LiAlH₄-LiBH₄ pellet is 65% of theoretical capacity in the temperature range of 80–475 °C, while that of nanoconfined LiAlH₄-LiBH₄ pellet is up to 80% at lower temperature of 100–400 °C. Besides, nanoconfined LiAlH₄-LiBH₄ pellet shows significant reduction of activation energy (ΔE_A up to 69 kJ/mol H₂) as compared with milled sample. Significant enhancement of volumetric hydrogen storage capacity up to 64% (from 32.5 to 53.3 gH₂/L) is obtained from nanoconfined LiAlH₄-LiBH₄ pellet. Hydrogen content released and reproduced of nanoconfined LiAlH₄-LiBH₄ pellet are 67 and 50% of theoretical capacity, respectively, while those of milled LiAlH₄-LiBH₄ pellet are only 30 and 10%, respectively. Moreover, upon four hydrogen release and uptake cycles, nanoconfined LiAlH₄-LiBH₄ pellet can preserve its shape with slight cracks, suggesting good mechanical stability during cycling. Curvatures and fibrous structure woven on one another of ACNF in nanoconfined LiAlH₄-LiBH₄ pellet not only favor hydrogen permeability through pellet sample during de/rehydrogenation, resulting fast kinetics, but also reinforce the pellet shape during cycling under high temperature and pressure condition.     

A computational study of LiBH4 clusters and enhancement of their hydrogen storage by excess electrons

The electronic structures and energies of neutral and anionic (LiBH₄)_x clusters (x = 1 – 5) have been systematically studied by using density functional theory with the B3LYP/6‐311++G(d, p) level. For investigating the importance of excess electrons on hydrogen storage capacity, the interactions between hydrogen atoms and the anionic (LiBH₄)_x clusters are also examined. The calculated formation energies of the anionic clusters show that the anionic clusters have a high thermal stability. It is found that hydrogen atoms are adsorbed on the anionic (LiBH₄)_x clusters chemically with adsorption energies in the range of ?69.13 – ?153.73 kcal/mol. The hydrogen storage capacity can be improved from 18.51% to 19.26 – 22.12% in weight percent depending on the size of various anionic (LiBH₄)_x clusters. Our calculation results show that the existence of excess electrons on the (LiBH₄)_x clusters can enhance the hydrogen storage capacity. The Mulliken charge analysis was performed to illustrate the interactions between H atoms and the anionic (LiBH₄)_x clusters. Copyright © 2016 John Wiley & Sons, Ltd.     

Excellent catalysis of MoO3 on the hydrogen sorption of MgH2

To improve the hydrogen sorption kinetics of MgH₂, the MoO₃ nanobelts were added into MgH₂ by mechanical milling, leading to fine distribution of MoO₃ in the MgH₂ matrix. Compared to uncatalyzed MgH₂, the hydriding and dehydriding rates of MoO₃-catalyzed MgH₂ were significantly improved. The MgH₂ doped with 2 mol% MoO₃ exhibited fast dehydrogenation without activation, and the initial dehydrogenation amount of 5 wt% could be reached within 900 s at 300 °C. The dehydrogenation apparent activation energy is decreased down to 114.7 kJ/mol. The excellent catalytic effect of MoO₃ originates from its specific role as fast hydrogen diffusion pathways. In the hydrogenation process, the MoO₃ transformed to MoO₂, resulting in the fading of catalytic activity.     

Confinement effects on structural,electronic properties and dehydrogenation thermodynamics of LiBH4

Confinement effect on the structural, electronic and thermodynamic properties of LiBH₄ is investigated by density functional theory. The thermodynamically and dynamically stable confinement structure is testified to be γ-LiBH₄@C₃₁Ti according to the adsorption energy and vibrational frequency calculations. The tridentate structure formed by [BH₄]⁻ and Li⁺ in the unconfined LiBH₄ changes into bidentate structure in γ-LiBH₄@C₃₁Ti. We observe that both the occupied and unoccupied states of H 1s, B 2s, B 2p, Li 2s, and Li 2p orbitals in the partial DOSs of γ-LiBH₄@C₃₁Ti shift to high energy level and the splits of DOS peaks occur at the states of H 1s, B 2p, and Li 2p orbitals. Different from the first-step decomposition reaction of LiBH₄, the one for γ-LiBH₄@C₃₁Ti changes into 2LiBH₄@C₃₁Ti → 2LiH + 2B@C₃₁Ti + 3H₂. Moreover, the reaction enthalpy for the first-step decomposition reaction of γ-LiBH₄@C₃₁Ti decreases to 5.864 eV, which is smaller than that (17.204 eV) of LiBH₄. According to the hydrogen removal energy calculations, we observe that the confinement effects make the removal of the first and second hydrogen atoms in γ-LiBH₄@C₃₁Ti easy.     

Comparative study on dehydrogenation of bulky,branched and polycondensed naphthenes for hydrogen storage in microwave and thermal modes

Microwave radiation can effectively heat chemical reactors in which bulky, branched and polycondensed naphthenes convert into the appropriate aromatics at atmospheric pressure. Two types of catalysts, traditional Pt/C and bifunctional Ni–silica–alumina, were used for dehydrogenation of naphthenes under microwave radiation. From the dehydrogenation reaction in microwave mode and in conventional heating mode, it was found that the catalytic activity in microwave mode increased more greatly than that in conventional heating mode at the same reaction temperatures. Such an effect may result from the fact that the temperature of the metal particles (Pt, Ni) in microwave mode is higher than the average temperature of the catalyst bed in thermal mode.     

Effect of crystallite size of Al on the reversible hydrogen storage of NaAlH4 and few aspects of catalysts and catalysis

NaAlH₄ has been catalyzed by MWCNT, Ce rich mischmetal (Mm), MmNi₅ and TiO₂ catalysts. The general aspect which relates every catalyst with the hydrogen storage capacity of NaAlH₄ system has been verified through XRD analysis. Interesting features like chemical reduction, grain size variation, hydrogenation/dehydrogenation and phase transformation of the catalytic species are noticed. In the case of reversible hydrogen uptake, an interesting relationship exists between the restored hydrogen capacity, crystallite size of Al (desorbed in the dehydrogenation reaction) and the applied hydrogen pressure. Thus, as far as the reversible hydrogen storage in NaAlH₄ is concerned, the mysterious role of catalyst seems to be a process of restricting the size of Al in a narrow range. The factors considered for analyzing this claim are discussed in detail.     

Catalytic effects of FGO-Ni addition on the dehydrogenation properties of LiAlH4

LiAlH₄ is an ideal hydrogen storage material with a theoretical hydrogen storage capacity of 10.6 wt%. In order to reduce the hydrogen release temperature and increase the hydrogen release amount of LiAlH₄, multilayer graphene oxide and nickel (FGO-Ni) composite catalyst were prepared by physical ball milling and doped into LiAlH₄. The effect of FGO-Ni composite catalyst on the dehydrogenation performance of LiAlH₄ was studied by pressure-composition-temperature apparatus, scanning electron microscope (SEM) and X-ray diffractometer. The results show that, compared with pure LiAlH₄, the hydrogen release time of LiAlH₄ doped with 9 wt%FGO-3wt%Ni is obviously shortened about 90min at 150 °C and the hydrogen release amount of LiAlH₄ doped with 9 wt%FGO-3wt%Ni also increased 1.8 wt%. Importantly, the dehydrogenation amount of LiAlH₄ (9 wt%FGO)-3wt% could reach 4 wt% at 135 °C which was 4 times higher than that of the pure LiAlH₄. At the same temperature, the hydrogen release of pure LiAlH₄ was only 0.84 wt%. In contrast, doping FGO-Ni composite catalyst reduces the hydrogen release temperature of LiAlH₄ and weakens the hydrogen release barrier. Forthermore, SEM results showed that doping FGO-Ni reduced the agglomeration between LiAlH₄ particles and increased the specific surface area of the sample, which improving the hydrogen release properties of LiAlH₄.     

Tuning the reaction mechanism and hydrogenation/dehydrogenation properties of 6Mg(NH2)29LiH system by adding LiBH4

The hydrogen storage properties of 6Mg(NH₂)₂9LiH-x(LiBH₄) (x = 0, 0.5, 1, 2) system and the role of LiBH₄ on the kinetic behaviour and the dehydrogenation/hydrogenation reaction mechanism were herein systematically investigated. Among the studied compositions, 6Mg(NH₂)₂9LiH2LiBH₄ showed the best hydrogen storage properties. The presence of 2 mol of LiBH₄ improved the thermal behaviour of the 6Mg(NH₂)₂9LiH by lowering the dehydrogenation peak temperature nearly 25 °C and by reducing the apparent dehydrogenation activation energy of about 40 kJ/mol. Furthermore, this material exhibited fast dehydrogenation (10 min) and hydrogenation kinetics (3 min) and excellent cycling stability with a reversible hydrogen capacity of 3.5 wt % at isothermal 180 °C. Investigations on the reaction pathway indicated that the observed superior kinetic behaviour likely related to the formation of Li₄(BH₄)(NH₂)₃. Studies on the rate-limiting steps hinted that the sluggish kinetic behaviour of the 6Mg(NH₂)₂9LiH pristine material are attributed to an interface-controlled mechanism. On the contrary, LiBH₄-containing samples show a diffusion-controlled mechanism. During the first dehydrogenation reaction, the possible formation of Li₄(BH₄)(NH₂)₃ accelerates the reaction rates at the interface. Upon hydrogenation, this ‘liquid like’ of Li₄(BH₄)(NH₂)₃ phase assists the diffusion of small ions into the interfaces of the amide-hydride matrix.